Focal Mechanisms Explained: What are those “beach balls”?

Summary

Key Points

• Focal mechanisms, or beach balls, are graphical representations used by seismologists to depict the faulting motions that cause earthquakes. 0:09
• For a strike-slip earthquake, P-wave arrivals show compression in two diagonally opposite quadrants and dilatation in the other two, separated by nodal planes. 1:22
• The observed P-wave first-motion pattern can be ambiguous, meaning a single beach ball can represent multiple possible fault orientations and slip directions. 1:33
• Geological context is crucial for determining the actual fault plane from the two nodal planes shown in a focal mechanism. 1:50
• To visualize P-wave radiation in three dimensions, seismologists imagine a focal sphere surrounding the earthquake's hypocenter. 2:51
• These focal mechanisms are determined by analyzing the direction of the first arriving P waves at seismic stations. 3:43
• Normal faults, caused by extensional forces, produce a focal mechanism with compressions radiating outwards and dilatations in the center on the lower hemisphere of the focal sphere. 5:04
• The orientation and shape of the boundaries between compressional and dilatational quadrants in a focal mechanism provide signatures for different types of faulting. 5:04
• Thrust faults, caused by compressional forces, result in a focal mechanism with dilatations radiating outwards and compressions in the center, often described as a 'cat's eye' pattern. 6:09